1. Field of the Invention
The present invention relates to the field of electric motor control, in particular the field of calibrating an energy conversion device.
The present invention can be applied in fields where an electric motor must apply a precise torque to an object, for example in the field of screwing, in particular of screwcaps.
2. Description of the Prior Art
The document EP 0 524 196 discloses a screwing device including a stopper cone, a motor for driving rotation of the stopper cone, a switching device and a power supply. A control device includes a torque sensor adapted to measure an instantaneous drive torque, a comparator for comparing the instantaneous torque with a predetermined closure torque, and a sensor responsive to the angle of rotation of the cap receiving member adapted to be activated only if the instantaneous torque reaches the predetermined value. Thus a drive torque is applied to a cap to screw it onto a container, the instantaneous torque applied to the cap is measured and compared with a closure torque of predetermined value, and the rotation angle of the cap is measured only if the instantaneous torque reaches the predetermined value.
However, the presence of a torque sensor and an angle sensor makes the device somewhat complicated without guaranteeing very reliable and convenient closure of the containers by the caps.
The present invention proposes, in particular, an improved control device and method enabling a very precise torque to be applied.
The present invention proposes, in particular, an economical calibration method reducing the duration of the maintenance calibration operations carried out at regular intervals during the life of an electric motor.
The device in accordance with one aspect of the invention is for controlling an electric motor and includes means for measuring the current Im drawn by the motor, means for measuring the torque Cm supplied by the motor, power supply means for the motor, and an initialization unit capable of generating a torque set point Cc and of receiving and processing information relating to the current Im and the torque Cm to deduce therefrom a relationship between the set point Cc, the torque Cm and the current Im in order to be able to determine the torque Cm from the set point Cc and the current Im.
The means for measuring the torque Cm supplied by the motor are advantageously removable. They can be removed after calibration, the torque Cm being controlled in accordance with the set point Cc and verified by means of the current Im.
In one embodiment of the invention the device includes a calculation unit associated with memory means and a user interface, a first data bus between the initialization unit and the power supply means of the motor, and a second data bus between the initialization unit and the calculation unit.
In one embodiment of the invention the means for measuring the torque Cm are provided with a removable calibrator for calibrating the torque measuring means.
The device can be used to control a plurality of motors and can include a corresponding number of current measuring means and motor power supply means.
The invention also relates to an electromechanical system including a device as defined hereinabove. The system can be a machine for applying a particular torque to an object, for example a screwing, drilling, tapping, capping, etc. machine.
The method according to one aspect of the invention is for controlling an electric motor. An initialization unit generates a torque set point Cc addressed to power supply means of the motor. Current measuring means measure the current Im drawn by the motor. Torque measuring means measure the torque Cm supplied by the motor. The initialization unit receives and processes information relating to the current Im and the torque Cm to deduce therefrom a relationship between the set point Cc, the torque Cm and the current Im in order to be able to determine the torque Cm from the set point Cc and the current Im.
The torque measuring means are preferably installed beforehand and then removed after measuring at least one value of the torque Cm. The same torque measuring means can be used for several electric motors.
In one embodiment of the invention the initialization unit generates three torque set point values Cc, the current measuring means measure n values of the current Im drawn by the motor, where n is greater than 2, the torque measuring means measure n values of the torque Cm supplied by the motor, and the initialization unit receives and processes n values relating to the current Im and the torque Cm to deduce therefrom a relationship between the set point Cc, the torque Cm and the current Im in order to be able to determine the torque Cm from only the set point Cc and the current Im.
In one embodiment of the invention the initialization unit calculates three coefficients a, b, c of a second order equation relating to the set point Cc and the torque Cm and three coefficients axe2x80x2, bxe2x80x2, cxe2x80x2 of a second order equation relating the current Im and the torque Cm. Using a second order equation is very suitable when using electric motors operating over a particular range outside their area of linear operation. A higher order equation or any other type of mathematical equation can be used for operation over a very wide range.
The computer program in accordance with one aspect of the invention includes program code means for implementing the steps of the method defined hereinabove.
The medium in accordance with one aspect of the invention can be read by a device for reading program code means stored therein and which are adapted to implement the steps of the method defined hereinabove.
One particular embodiment of the invention is shown in the accompanying drawings.
FIG. 1 is a diagrammatic view of a motor, a torque measuring system and a calibrator.
FIG. 2 is a diagrammatic view of a motor control device and its environment.
FIG. 3 is a functional block diagram of a motor control device.
FIG. 4 is a process step flowchart.
FIG. 5 is a block diagram of a torque measuring system.